1. Field of the Invention
This invention relates to the art of catalytic cracking of hydrocarbons, and in particular to methods of inhibiting on zeolite catalysts the detrimental effects of contamination by metals, particularly nickel, which are contained in the hydrocarbon feedstock.
Major metal contaminants that are found in Fluid Catalytic Cracker (FCC) feedstocks include nickel, vanadium, iron, copper and occasionally other heavy metals. The problems associated with metal contamination, particularly nickel, during the catalytic cracking of hydrocarbons to yield light distillates such as gasoline are documented in Oil & Gas Journal of July 6, 1981 on pages 103-111 and of Oct. 31, 1983 on pages 128-134. The problems associated with vanadium metal contamination are described in U.S. Pat. No. 4,432,890 and German Patent No. 3,634,304. The invention herein represents an innovation and improvement over those processes set forth and claimed in U.S. Pat. No. 4,432,890 and German Patent No. 3,634,304.
It is well known in the art that nickel significantly increases hydrogen and coke and can cause decreases in catalyst activity. Vanadium primarily decreases activity and desirable gasoline selectivity by attacking and destroying the zeolite catalytic sites. Its effect on the activity is about four times greater than that of nickel. Vanadium also increases hydrogen and coke, but at only about one fourth the rate of nickel.
The reducing atmosphere of hydrogen and carbon monoxide in the cracking zone reduces the nickel and vanadium to lower valence states. The nickel is an active dehydrogenating agent under these circumstances, increasing hydrogen and coke which also leads to a small decrease in conversion activity.
Vanadium has been shown to destroy active catalytic sites by the movement of the volatile vanadium pentoxide through the catalyst structure. Lower oxides of vanadium are not volatile and are not implicated in the destruction of catalyst activity. In the cracking zone, lower oxides of vanadium will be present and vanadium pentoxide will be absent. Thus in the cracking zone, fresh vanadium from the feedstock will not reduce activity. When the lower valence vanadium compounds enter the regenerator where oxygen is present to combust the coke, the vanadium compounds are oxidized to vanadium pentoxide which then can migrate to active sites and destroy the active sites, leading to a large reduction in activity and selectivity, particularly gasoline.
An increase in hydrogen and coke due to contaminant metals translates to a decrease in yields of desirable products such as gasoline and light gases (propane/butanes). Also, increases in hydrogen yield require extensive processing to separate the cracked products and can result in operation and/or compressor limitations.
While the coke that is produced during the catalytic cracking process is used to keep the unit in heat balance, increases in coke yields mean increased temperatures in the regenerator which can damage catalysts by destroying the zeolitic structures and thus decrease activity.
As activity is destroyed by contaminant metals, conversion can be increased by changing the catalyst to oil ratio or by increasing the cracking temperature, but coke and hydrogen will also be increased in either case. For best efficiency in a FCC unit, the activity should be kept at a constant level.
However, as vandium is deposited on the catalyst over and above about a 3,000 ppm level, significant decreases in activity occur. Passivators have been used to offset the detrimental effects of nickel and of vanadium.
Numerous passivating agents nave been taught and claimed in various patents for nickel. Some examples include antimony in U.S. Pat. Nos. 3,711,422, 4,025,458 4,111,845, and sundry others; bismuth in U.S. Pat. Nos. 3,977,963 and 4,141,858; tin in combination with antimony in U.S. Pat. No. 4,255,287; germanium in U.S. Pat. No. 4,334,979; gallium in U.S. Pat. No. 4,377,504, tellurium in U.S. Pat. No. 4,169,042; indium in U.S. Pat. No. 4,208,302; thallium in U.S. Pat. No. 4,238,367; manganese in U.S. Pat. No. 3,977,963; aluminum in U.S. Pat. No. 4,289,608, zinc in U.S. Pat. No. 4,363,720; lithium in U.S. Pat. No. 4,364,847; barium in U.S. Pat. No. 4,377,494; phosphorus in U.S. Pat. No. 4,430,199; titanium and zirconium in U.S. Pat. No. 4,437,981; silicon in U.S. Pat. No. 4,319,983; tungsten in U.S. Pat. No. 4,290,919; and boron is U.S. Pat. No. 4,295,955.
Examples of vanadium passivating agents are fewer, but include tin in U.S. Pat. Nos. 4,101,417 and 4,601,815; titanium, zirconium, manganese, magnesium, calcium, strontium, barium, scandium, yttrium, lanthanides, rare earths, actinides, hafnium, tantalum, nickel, indium, bismuth, and tellurium in U.S. Pat. Nos. 4,432,890 and 4,513,093; yttrium, lanthanum, cerium and the other rare earths in German 3,634,304.
In general, the passivating agents have been added to the catalyst during manufacture, to the catalyst after manufacture by impregnation, to the feedstock before or during processing, to the regenerator, and/or any combination of the above methods.
2. General Description of the Invention
It was discovered that when a zeolite catalyst contaminated with metals, including nickel, is treated with cerium compounds, the hydrogen-forming property of the nickel was mitigated to a great extent.
While cerium passivates vanadium, it was quite unexpectedly found that cerium also passivates the adverse effects of nickel.
U.S. Pat. Nos. 4,432,890 and 4,513,093 teach that numerous metallic compounds (titanium, zirconium, manganese, magnesium, calcium, strontium, barium, scandium, yttrium, lanthanides, rare earths, actinides, hafnium, tantalum, nickel, indium, bismuth, and tellurium act as vanadium passivators. German Patent No. 3,634,304 claims that yttrium, lanthanides, cerium, and other rare earth compounds passivate the adverse effects of vanadium. In the '890 patent, only titanium was used on an FCC catalyst to show the effects of the various claimed metals on passivating vanadium. Cerium was not specifically mentioned. In each of these patents, nickel was not added to the catalyst undergoing testing and so the effects on hydrogen-make by nickel with cerium passivation could not be observed. In addition, the only vanadium levels tested in these two patents were 5,500 and 3,800 ppm, respectively. Although nickel and vanadium contamination of FCC catalysts is discussed in great depth in the art and in the same context, it is equally clear from the specifics of the art, that each represents its own separate problem as well as solution. It is not evident or expected that any treatment for vanadium would also be effective for nickel or vice-versa.
It is well documented in the art that a certain level of vanadium is necessary on the catalyst to observe a loss of catalyst activity. This level varies with the type of catalyst. In one report the level of vanadium below which catalyst activity is not degraded is 1,000 ppm for that catalyst (see the newsletter Catalagram published by Davison Chemical in 1982, Issue Number 64). In another article (R. F. Wormsbecher, et al., J. Catal., 100, 130-137(1986)), only above 2000 ppm vanadium are catalyst activity and selectivity lost. Other catalysts such as metal resistant catalysts need high levels (above about 3000 ppm) of vanadium where loss of catalyst activity can be observed (Oil & Gas Journal, 103-111, July 6, 1981). From these articles, it can be seen that not all catalysts are significantly affected by lower levels of vanadium contaminant.
Thus, the treatment of specific catalysts containing less than a significant level of vanadium would show very small to insignificant changes in activity on addition of cerium. However, the practical effects of nickel can be observed at levels as low as about 300 ppm, with the amount of hydrogen and coke increasing proportional to the amount of nickel present.